Dynamic Nuclear Polarization (DNP) has emerged as a powerful strategy to increase the sensitivity of NMR experiments on a wide range of biological systems by transferring the large polarization of electron spins (EPR) to nuclear spins (NMR). Crucial to the successful implementation of DNP in conjunction with magic angle spinning (MAS) has been the development of gyrotrons and NMR probes, instrumentation used to perform DNP. DNP currently enhances the sensitivity of NMR experiments on membrane proteins by a factor of about 50 and on model systems up to about 120 at 9 Tesla.
The larger gyromagnetic ratio of electron spins compared to proton spins, lower temperatures, and faster recycle delays all combine to potentially increase the NMR sensitivity by a factor of 360,000. The associated experimental averaging time may decrease by a factor of 133 billion. Transferring 100% of the polarization from the electron spins and cooling samples to 5 K to achieve the theoretical gains poses an ongoing challenge.
The microwave source (usually a gyrotron) used in contemporary DNP experiments is left locked on the same frequency for continuous-wave operation during the entire experiment. This is because although DNP gyrotrons have high microwave power output levels (>10 W), they have not yet been tuned on a fast timescale in existing magnetic resonance experiments.
Current MAS DNP technology may experience difficulty achieving sufficient control of EPR spins. Only a fraction of the 1 GHz broad nitroxide lineshape can be covered with a non-tunable 1 MHz γB1 microwave field of about 200 GHz that exerts control over the EPR spins. Others have not been able to use EPR spin labels on peptides for DNP because of extensive paramagnetic broadening. Therefore, there is a need for a frequency agile gyrotron microwave source that can output short pulses to not only sweep-through the EPR linewidth, but also to control all of the EPR spins simultaneously with a broad excitation bandwidth. At the same time, there is a need to increase the γB1 microwave field strength by about 3 orders of magnitude (from about 1 MHz to about 1 GHz).